2.1. Navigating the File System

In the previous tutorial, you experienced the pwd (print working directory) command and were provided a brief introduction to absolute paths. In Unix, a directory file or directory is the same concept as a folder in other operating systems; that is, in most cases the words “directory” and “folder” are interchangeable. Your current working directory is the directory in which you are currently executing commands. There are other directories, and in Unix, they are all organized under a top-most directory called /, representing the root of the file system. The macOS and Linux operating systems organize their file systems the same way. If you are coming from a Windows background, then it is reasonable to compare / to the C: drive.

For this part of the tutorial, we are going to assume that you have some specific files in your home directory. To make sure that you have these files, you should execute the command below. The command may take a few seconds to run as it involves downloading some files from the Internet. You don’t need to memorize or understand the command below; you just need to execute it to proceed with the tutorial.

curl -s -L https://git.io/fjbdg | bash

2.1.1. ls, ls PATH, tree PATH

To list the files in your current working directory, use the ls (list) command. It is common for ls to differentiate between directories and regular files in its output using different colors.

ls

• How many directories and regular files are in your home directory?

The ls (list) command is not limited to the current working directory. You can tell ls to list the files in some other directory by supplying a path to that directory as a command-line argument.

ls cs1302-unix

• The paths used in the examples above are relative paths. Instead of describing the traversal path starting from /, they describe the traversal path starting from the current working directory. The ls command also works with absolute paths. What is the command to list the files in your home directory using an absolute path?

While ls is nice and probably one of the most used Unix commands of all time, it doesn’t let us see the whole picture without us issuing repetitive ls commands. To see the bigger picture, use the tree command, which lists the contents of a directory in a tree-like format. If this command doesn’t work for you, please go back and make sure you have completed the previous tutorial and all the required setup steps.

tree cs1302-unix

Test Yourself

• Which files are contained in the cs1302 directory?

• What about the books directory?

Command	Description
ls	List contents of the current working directory.
ls PATH	List contents of the last directory in the provided PATH (e.g., c in a/b/c or /a/b/c).
tree PATH	List contents of the last directory in the provided PATH and its subdirectories, in a tree format.

2.1.2. ls -l, ls -lh

By default, the ls command displays the contents of a directory using its “simple” output format. A “long” output option can be specified by supplying a -l as a command-line argument. In addition to the name of each file, additional information is included in the “long” output format, as described below in the next few examples.

ls -l cs1302-unix/books

Mode		User	Group	Size	Timestamp	Name
-rw-r--r--.	1	mepcott	myid	1276233	Feb 28 10:45	moby_dick.txt
-rw-r--r--.	1	mepcott	myid	799645	Feb 10  2021	pride_and_prejudice.txt
-rw-r--r--.	1	mepcott	myid	607430	Jun  9 12:45	sherlock_holmes.txt

ls -l cs1302-unix

Mode		User	Group	Size	Timestamp	Name
drw-r-xr-x.	2	mepcott	myid	85	Aug 17 17:39	books
drw-r-xr-x.	4	mepcott	myid	46	Aug 17 17:39	notes
-rw-r--r--.	1	mepcott	myid	9	Aug 17 17:39	README.md

You don’t need to understand most of the columns in a long listing produced by ls -l at this point in time. The takeaway is that adding a command-line argument like -l changes the behavior of ls without needing to ask the user for any additional input.

You can supply -h as a command-line argument to ls -l to enable the display of human-readable units for file sizes. Many Unix programs like ls allow you to combine single character (short) options into a single command-line argument when entering the command at the shell prompt. All three examples below produce the same output.

ls -l -h cs1302-unix/books

ls -h -l cs1302-unix/books

ls -lh cs1302-unix/books

Test Yourself

• What command can be used to see the sizes of the files in a directory?

• What is second way to accomplish the same task using the same options?

Command	Description
ls -l	ls uses long listing format
ls -l -h	ls uses long listing format with human-readable size units

2.1.3. cd PATH, cd -, cd ~, cd

So far, you know how to list the files in your home directory, your current working directory, and other directories; however, all of the examples so far have utilized absolute or relative paths in a way that kept you in your home directory. It’s time for that to change, or rather it’s time for your current working directory to change. This can be done using the cd (change directory) command.

As you navigate to various directories in this section, try typing pwd and ls in each directory. Note how the output of each command changes as you change directories. That’s because commands are always executed in the context of the current directory.

cd cs1302-unix

cd notes/cs1302

If you to change to the directory you were last in prior to your current working directory, then supply - (minus sign) as a command-line argument to cd.

cd -

On most Unix systems, ~ (tilde) is an alias for the absolute path of your home directory. While you can use it with cd to change directly to your home directory, it’s more commonly used to change to directories nested under your home directory.

cd ~/cs1302-unix

If you use the cd command without supplying any command-line arguments, then it changes to your home directory. How convenient!

cd

Test Yourself

• What two commands (or variations of the same command) can always be used to take you to your home directory?

• How can you tell if a path is absolute or relative?

• How can you convert a relative path to an absolute path (and vise-versa)?

Command	Change to
cd	home directory
cd -	last previous working directory
cd a/b/dest	dest, assuming a is in the current directory and a/b/dest is a valid relative path
cd /a/b/dest	dest, assuming /a/b/dest is a valid absolute path
cd ~/a/b/dest	dest, assuming a is in your home directory and ~/a/b/dest is a valid path

2.1.4. ls -a, cd ..

The default behavior of ls is to ignore files starting with . (hidden files). To force ls to not ignore hidden files, supply the -a (all) option as a command-line argument.

ls -a

Every directory on a Unix system has two special hidden files named . and ...

File	Description
.	path alias for the directory itself
..	path alias for parent of the directory (parent directory)

Remember, the default behavior of ls is to list the contents of the current working directory. If we supply . as a path to ls, then it does the same thing.

ls .

You can supply .. as a path for ls to list the contents of the current working directory’s parent (i.e., its parent directory).

ls ..

You can supply .. as a path to cd to change to go up one directory; that is, change to the parent directory.

cd ..

You can even go up two directories using ../...

cd ../..

Command	Description
ls -a	ls shows hidden files (those that start with .)
ls .	ls lists the contents of . (current directory); same as ls with no arguments
ls ..	ls lists the contents of .. (parent directory)
cd ..	cd changes working directory to .. (parent directory; “up one”)
cd ../..	cd changes working directory to ../.. (parent of parent directory; “up two”)

Test Yourself

• What is the difference between . and ..?

• How can you leverage .. to go up a directory tree by three directories (i.e., instead of up by one or up by two) using a single command?